Replication stress can be defined as any condition in which the number of replication origins, the timing of origin firing, and/or replication fork speed are significantly altered relative to normal S phase. Under these conditions, cells accumulate unresolved replication intermediates, failing to complete DNA replication. Recent studies demonstrate that replication stress is widespread in a variety of cancers; therefore its exploitation for therapies is an exciting new area to explore. To this end, the majority of current investigations focus on the DNA damage response pathways as major therapeutic targets. In this application, we propose dormant origins as a new target to exploit replication stress in cancer, as increasing evidence indicates that cancer cells rely heavily on dormant origins to override replication stress and sustain their proliferation. All replication origins are licensed by loading of the MCM2-7 proteins onto chromatin during the late M and early G1 phases. In the subsequent S phase, cells use only a small fraction (5~10%) of chromatin-bound MCM2-7 proteins for the assembly of active replicative helicases to unwind DNA and initiate DNA synthesis. The remaining excess MCM2-7 proteins license dormant origins that can act as backups to rescue stalled replication forks and/or compensate for slow fork progression. While such roles of dormant origins are important even in normal S phase, they are used far more frequently under replication stress that increases the number of stalled forks. Possibly reflecting their over-reliance on dormant origins, cancer cells generally show higher levels of MCM2-7 expression as cancer progresses. Given these properties of dormant origins, we hypothesize that reducing their number will contribute to the selective killing of cancer cells exhibiting replication stress. For a reduction of dormant origins, we will use either RNA-interference mediated depletions of the MCM2-7 proteins or the Mcm4chaos3 homozygous background in which the number of dormant origins are reduced to ~50 % of what is observed in wildtype mice. As a cancer model, we will use the in vitro and in vivo systems for inducible FGFR1 (iFGFR1) activation in breast cancer, because our preliminary data demonstrate that iFGFR1 activation causes robust replication stress. As breast cancers with FGFR abnormalities often develop therapeutic resistance, the development of new therapies targeting dormant origins could benefit patients with such diseases. Our specific aims are as follows: 1) Prove that a reduction of dormant origins impairs the completion of DNA replication upon FGFR activation, 2) Test if a reduction of dormant origins blocks iFGFR1-induced cellular proliferation. We will also determine the efficacy of co-targeting dormant origins with pathways that mediate FGFR-induced survival. Novel aspects of this proposal are not only to test the targeting of dormant origins to exploit replication stress, but also to investigate the mechanism by which aberrant activation of FGFR induces replication stress for the development of combined therapies to achieve the specific killing of cancer with FGFR abnormalities.